The present invention relates to cleaning of gas turbine engine-run airfoils such as turbine blades and vanes and, more particularly, to cleaning of oxides and dirt from internal cooling air passages of airfoils by immersion in an inorganic hydroxide cleaning medium for a short time.
During service in a gas turbine engine, both the internal and external surfaces of airfoils, such as nickel or cobalt base superalloy turbine blades and vanes, suffer oxidation from environmental attack. In addition, dirt typically accumulates in the internal cooling air passages of the airfoils. The build-up of foreign material (oxidation products and/or dirt) inside the cooling air passage results in their decreased effectiveness and a resultant increase in the operating temperature of the airfoil, and ultimately a decrease in service life of the airfoil.
Gas turbine engine airfoils typically are periodically refurbished to extend their useful service life. Before the engine run airfoils are refurbished, they are cleaned to remove oxides and dirt from the internal and external surfaces of the airfoil and/or stripped of any existing protective coatings prior to recoating.
Several cleaning processes have been developed to clean internal cooling passages of gas turbine engine airfoils. U.S. Pat. No. 5,507,306 describes a process where a KOH solution at elevated temperature is sprayed under pressure into the airfoil internal cooling passages. U.S. Pat. No. 4,439,241 discloses a high pressure autoclave cleaning process using a KOH solution to remove internal deposits from airfoils. Most of these cleaning processes involve relatively complex equipment, which is expensive to build and maintain. Moreover, many of the cleaning processes are relatively slow to clean the internal passages of oxides and dirt, requiring long processing time and increasing the cost to refurbish airfoils.
It is an object of the present invention to provide a method for cleaning internal passages of an engine-run gas turbine engine airfoils that overcomes the above disadvantages.
The present invention provides a chemical cleaning method for cleaning an internal passage of an engine-run gas turbine engine airfoil wherein the airfoil is immersed in an inorganic hydroxide medium at superambient temperature and atmospheric pressure for a short time to remove accumulated foreign material, such as oxides and dirt, from the surface of the internal passage without adversely chemically attacking the airfoil, which may comprise a nickel or cobalt base superalloy in the event the airfoil is not protectively coated or a protective coating in the event the airfoil is protectively coated prior to engine service. Preferably, the hydroxide medium comprises molten KOH. Immersion time of the engine-run airfoil in the hydroxide medium preferably does not exceed 360 minutes and even more preferably does not exceed 10 minutes. For purposes of illustration and not limitation, the inorganic hydroxide medium can comprise a 90 weight % KOH and 10 weight % water solution at 575 degrees F. disposed at atmospheric pressure with immersion times not exceeding 10 minutes.
The present invention provides a refurbished gas turbine engine run airfoil having a cleaned internal passage surface that can be coated to form a protective coating thereon. The coating may include distinct oxide regions therein that formerly were present as sub-surface internal oxide regions in the cleaned airfoil prior to coating.
The above and other objects and advantages of the present invention will become more readily apparent from the following drawings taken in conjunction with the following detailed description.